Cygnus A (3C 405; \(z =0.0561\) ) is the nearest and most powerful example of a classical double radio galaxy, and as such, is considered to be the archetype of this class of radio source. Cygnus A is one of the first objects in which the hotspot radio spectra were seen to flatten towards low frequencies [1]}, but the physical cause of this spectral flattening has been a source of contention. [2]} performed a detailed multi-frequency analysis of Cygnus A at 4.5 arcsec resolution, based on imaging with the Very Large Array (VLA) at several frequencies between 327 MHz and 22 GHz, along with 3.0 arcsec resolution Multi-Element Radio Linked Network (MERLIN) imaging at 151 MHz taken by [1]}. At frequencies above 1 GHz, they found the spectra of the two secondary hotspots (A and D) to be well described by a broken power law of the form predicted by a continuous injection model, in which a power-law electron energy distribution is continuously injected into the hotspot, and radiative cooling acts to modify the electron distribution as it flows away from the hotspots and into the lobes. In addition, the injected particle spectrum was found to agree with the predictions from diffusive shock acceleration.
